This term from sociology refers to a household consisting of a mother, father, and a child or children.

By contrast, many families are more than just this nucleus. An extended family may include grandparents, aunts and uncles, cousins, or other relatives living in the same home. A blended family could also include a stepmother or stepfather, half-sisters and half-brothers, and so on.

{The word 'nuclear' here means 'central and elemental', not 'radioactive'. liveforever points out, however, that the term 'nuclear family' is also used in the media to refer to the nuclear club of nations with nuclear weapons.)